This invention pertains to drives for ferrite materials used to load microwave transmission lines and more particularly pertains to a blocking oscillator drive for such transmission lines.
Ferrite materials have been used to load waveguide, stripline and coaxial transmission lines for a considerable length of time. At first, the magnetic fields required by the ferrite materials to produce the desired effects were maintained by fixed external magnets. Some degree of temperature compensation in such an external magnet system was obtained by using pole pieces with the proper change of reluctance with temperature to strengthen or weaken the magnetic field as required with temperature changes.
Later, the advantages that could be obtained by quickly changing the magnetic field were realized, and various arrangements for doing so were developed. One way that this was done was to still utilize fields from magnets outside the transmission line structure, but substituting electromagnets in place of permanent magnets. These electromagnets were made of laminated silicon iron or of ferrite and powdered iron for higher frequency applications. One problem with such arrangements was that the eddy currents generated in the metal structure of the transmission line made it difficult to achieve switching speeds faster than about one millisecond.
The slow speed, and generally bulky and power-consuming equipment required for this external electromagnet configuration was overcome when it was discovered that it was possible to arrange the core in a closed toroidal configuration completely within the transmission line structure. This proved to be possible and practicable despite the fact that the fields in the core were no longer in one simple direction with respect to the passing signal wave, but had some components in all directions, and the magnitude of the fields varied much more through the toroid.
This new arrangement of a closed toroidal configuration resulted in a requirement for a small switching circuit to match the more compact overall structure. Since the winding must now be within the microwave pathway, it was necessary to keep it as simple as possible with as few number of turns as possible. In many cases two turns is almost the maximum possible. This low number of turns made it necessary for the switch to handle large currents to produce the required magnetic field, with 10 amps being typical.
Several different types of control circuits have been employed in the past to produce the very fast, high current and moderate voltage pulses desired to change the direction of the magnetic fields in the closed toroidal configuration. In one of these arrangements, an SCR is utilized to discharge a capacitor (or possibly a length of lumped constant transmission line). The problem with this approach is that the SCR is difficult to cut off, and there is a need to break off the current after the desired degree of magnetization is reached. This level is reached before the supply of current is exhausted, or certainly before the system energy is expended. The result is that there is a long decaying tail of current which prevents the core from settling to its correct rest state until the current is low enough, or else an involved circuit is required in order to cut off the SCR. The tendency is for such a system to be somewhat complex and slow.
Other prior art approaches have utilized a power transistor to turn on and break off the required current. The pulse used to control the transistor is usually a fixed length pulse, generated either internally or externally by a control system.
It should be appreciated that the current required to produce the desired magnetic field is large and that the winding on the toroidal core goes through impedance changes during switching, constituting somewhat of a nonlinear load. Since it is desired that the current build up quickly and cut off quickly, it is advantageous to have the main switch for the current, as well as the source of current, located right at the microwave device.
When the toroid is magnetized, the degree of magnetization determines either the differential phase shift produced in a phase shifter, or the exactness with which a junction circulator directs its energy into the desired path, so that precise control over the degree of magnetization is desirable. The degree of magnetization is determined not only by the exciting magnetic field due to the current, but by the internal structure and characteristics of the toroid material as well as by the temperature of the material.
Even the results of applying a given voltage pulse to a winding of the toroid (referred to as a "latch" winding) can produce a variety of results depending upon the magnitude of the voltage pulse and the length of time it is applied due to the non linearity of the latch wire's apparent impedance.
Under laboratory conditions, when the size and complexity of the associated equipment is of no importance, it is possible to plot the size of the voltage pulse required to produce the desired degree of magnetization in the form of a volt .sub.3/8 time product to maintain the desired performance over a wide variety of temperatures. Then, using a transistor driven by a precisely controlled pulse, which can be obtained from a well compensated monostable oscillator in integrated circuit form, a fairly constant voltage-time pulse can be obtained. The resulting circuit is, however, somewhat complicated because the monostable oscillator must be amplified without picking up width changes, and such a circuit usually does not have good amplitude accuracy, so some additional circuitry must be included to hold the base drive level constant. With respect to holding the base drive level constant, it is well known that a transistor switch should ignore base drive changes in level if it is saturated, but this is true only to a degree. This is one problem.
An additional problem in using a power transistor to switch on and off the current is the storage time of the transistor. This storage time tends to be quite temperature dependent in just the wrong direction; the on time of the transistor tends to shorten as the temperature decreases, just when it would be desired to have it remain the same or slightly increase. There are several well known techniques that can be used to minimize the storage time problem. These include applying reverse base voltage, holding the collector just short of saturation, or using transistors which inherently have a minimum amount of storage time. All these approaches, however, tend to complicate and increase the cost of the circuit considerably. Cost and complexity are very real considerations, especially for applications such as a no moving parts type of scanning antenna where hundreds of phase shifters are required.
There are other types of temperature compensation that could be used. An obvious one is temperature control of the device and/or the overall environment, but this is not always feasible or possible. Another approach is to change the voltage -- time latch wire pulse by using a temperature sensing element to vary the supply voltage or the width of the base drive pulse. This is quite costly and complex to implement because the variation of the storage time is not particularly predictable due to variations in the microwave ferrite core.